Communication of information is an endemic necessity of modern society. Communication of information is effectuated through operation of a communication system. Information is communicated between a sending station and a receiving station by way of a communication channel. The sending station, if necessary, converts the information into a form to permit its communication upon the communication channels. And, the receiving station, if necessary, operates upon detected indications of the information to permit operations to be performed thereupon to recover the information.
A wide variety of different types of communication systems have been developed and are regularly utilized to effectuate communication of information between sending and receiving stations. New types of communication systems have been, and continue to be, developed and constructed as a result of advancements in communication technologies.
An exemplary communication system is a radio communication system. In a radio communication system, the communication channel is defined upon a radio link extending between the sending and receiving stations. Communication systems implemented as radio communication systems are sometimes of reduced costs, relative to wire line counterparts. And, radio communication systems are amenable to implementation as mobile communication systems since radio links, rather than fixed, wire line connections, are utilized upon which to define communication channels used to communicate the information.
A cellular communication system is exemplary of a radio communication system that has achieved significant levels of usage. Cellular communication systems have been installed throughout significant parts of the populated portions of the world. Various cellular communication standards have been promulgated, setting forth the operational parameters of different types of cellular communication systems.
Generally, a cellular communication system includes a fixed network infrastructure that includes a plurality of fixed-site base transceiver stations. The fixed-site base transceiver stations are positioned at spaced-apart locations throughout a geographical area that is to be encompassed by the communication system. Each of the base transceiver stations defines an area, referred to as a cell, from which the cellular communication system derives its name.
The fixed network infrastructure of which the base transceiver stations form portions is coupled to a core network, such as a packet data backbone or public-switched, telephonic network. Communication devices, such as computer servers, telephonic stations, etc. are, in turn, coupled to the core network, or elsewhere, and are capable of communication by way of the network infrastructure and the core network.
Portable transceivers, referred to as mobile stations, communicate with the base stations by way of radio links forming portions of the electromagnetic spectrum. Use of the cellular communication system is permitted, typically, pursuant to a service subscription, and users, referred to as subscribers, communicate by way of the cellular communication system through utilization of the mobile stations.
Information communicated upon a radio link is susceptible to distortion as a result of non-ideal communication conditions. Other communication systems are analogously non-ideal, and communication of information in such other communication systems analogously also is susceptible to distortion. The distortion causes values of the information delivered to a receiving station to differ with the corresponding values of the information, when transmitted by the sending station. If the distortion is significant, the informational content of the information cannot be accurately recovered at the receiving station.
Fading caused by multi-path transmission distorts information communicated upon a communication channel. If the communication channel exhibits significant levels of fading, the informational content of the information might not be able to be recovered.
Various techniques are utilized to compensate for, or otherwise overcome, the distortion introduced upon the information during its communication upon a communication channel to the receiving station. Space diversity, for instance, is sometimes utilized. Space diversity is created through the use, at a sending station, of more than one transmit antenna transducer from which information is transmitted. Spacial redundancy is provided therefrom. The antenna transducers are typically separated by distances great enough to ensure that the information communicated by respective antenna transducers fades in an uncorrelated manner. And, receiving stations sometimes also utilize more than one receive antenna transducer, also typically separated by appropriate separation distances.
Communication systems that utilize both multiple transmitting antenna transducers and multiple receive antenna transducers are sometimes referred to as being MIMO (Multiple-Input, Multiple-Output) systems. Communications in an MIMO system provide the possibility that higher overall capacity of the system, relative to conventional systems can be achieved. Increased number of users are able to be serviced, or more data throughput is capable of being provided for each user.
The advantages provided through the use of space diversity are further enhanced if the sending station is provided with information about the state of the interface, interfacing the sending and receiving stations, i.e., the communication channel.
A sending station is not able to measure channel characteristics of the communication channel directly. Such measurements are possible only at a receiving station. In two-way communication systems, measurements made at the receiving station can be returned to the sending station to provide an indication to the sending station of the channel characteristics.
Communication systems that provide this type of information to a multiple-antenna sending station are referred to as being systems that provide closed loop transmit diversity. Communication channels extending from the network infrastructure of a cellular communication system to a mobile station are sometimes referred to as being down link, or forward link, channels. And, channels extending from the mobile station back to the network infrastructure are sometimes referred to as being uplink, or reverse link, channels.
The feedback information returned to the sending station, here at the network infrastructure, from the receiving station, here a mobile station, is used to select values of antenna weightings. The weightings are weighting values by which information signals provided to separate ones of the antenna transducers are weighted prior to their communication upon a communication channel to the mobile station. A goal is to weight the information signals applied to the antenna transducers in manners best to facilitate communication of the information to the receiving station. The values of the antennas weightings approach a conjugate of the subspace spanned by down-link channel covariance matrix. Estimation of the antenna weightings can be formulated as a transmission subspace tracking procedure. Several closed loop transmit diversity procedures are utilized. TxAA, Eigenbeam Former, and other techniques are sometimes utilized. Existing techniques, however, suffer from various deficiencies. For instance, a TxAA procedure fails to take into account a long-term covariance matrix of the communication channel in the selection of the antenna weightings. And, use of an Eigenbeam former technique is dependent upon the number of antenna transducers of the sending station. When the number of antenna transducers increases, the complexity of such a technique increases rapidly.
What is needed, therefore, is an improved manner by which to implement closed-loop transmit diversity, thereby to permit improved communications in a MIMO, or other, communication system.
It is in light of this background information related to radio communication systems that utilize space diversity that the significant improvements of the present invention have evolved.